WASHINGTON — Sen. Joseph Lieberman threatened yesterday to join Republicans in opposing health-care legislation if it permits uninsured people as young to 55 to purchase Medicare coverage.

Lieberman (I-Conn.), whose vote is critical to the bill’s prospects, expressed his opposition first in an interview with CBS, and more strongly later in a private meeting with Senate Majority Leader Harry Reid, according to Democratic officials. Reid (D-Nev.), who is hoping to pass the legislation by Christmas, needs 60 votes to avoid a filibuster, and has been counting on Lieberman to provide one.

But on CBS, Lieberman said of the Medicare proposal, “I certainly would have a hard time voting for it because it has some of the same infirmities that the public option did. It will add taxpayer costs. It will add to the deficit. It’s unnecessary.”